The Future of Solar Geolocation Tracking is NOW

Working on FLightR, the package for analysis of data obtained from solar geolocation tracking devices, we were haunted by the unpleasant feeling of investing in technology which will soon be out of date. Until now solar geolocators have been popular in ornithological studies. This is because they’re small, light-weight (< 1/3 g) tracking devices that can be deployed even on miniature birds, such as swallows and warblers. They’ve also been the longest-lasting data loggers, with the most storage space and, of course, the most affordable ones.

Are Solar Geolocators Finished?

There are apparent drawbacks of using this technique though. To begin with, solar geolocation simply does not work for some species. You can’t use it to study birds living in dense tropical forests or in cavities, because of the light-pattern bias. For the same reason, it doesn’t provide fantastic results in light-polluted areas. Data from geolocators cannot be retrieved remotely, and this is why you need to have high recapture rates for the species you’re studying.

Using the data from geolocators is no piece of cake either. The devices don’t log ready-to-go geographic positions, they record light measurements accompanied by time stamps. Turning this information into geographic coordinates isn’t straightforward and requires careful pre-processing. The accuracy of those geographic coordinates is far from perfect (tens of km), so geolocation isn’t suitable for studies on movements of birds migrating short distances.

The main benefits of solar geolocators over GPS trackers were the size, storage, life-span and cost. But GPS tracking devices are becoming gradually smaller, more efficient and cheaper. And, of course they’ll always be more accurate than geolocators. It seems unavoidable that GPS trackers will replace solar geolocators in animal movement research sometimes soon. But not yet…

Solar Geolocators in Comparative Studies

One other major benefit of solar geolocation data is that they have been collected for the last two decades – they’re a tremendous resource for comparative studies. But, among the few examples of studies integrating tracks of multiple animal species in analysis, only one uses solar geolocation data.

Peer-reviewed publications based on solar geolocation data from Web of Science and Movebank.

We see two reasons for the geolocation data being rarely involved in comparative research. Firstly, just a tiny portion of geolocation data (330 tracks from 17 of more than 500 publications based on geolocation data) has been deposited at Movebank, a searchable web platform specialized on archiving tracking data. Accessibility of data is essential for meta-analyses. We hope (and expect) that using repositories like Movebank will become more popular and eventually help scientific collaboration and integration of multiple datasets in analytical frameworks.

Another stumbling stone for comparative studies is challenging data processing. Comparisons within and between species require transparent and reproducible analysis. Converting geolocation data into geographic coordinates has long been more of an art than an analytical process. The results of such hand-crafted processing are hard to compare.

Since the release of FLightR in 2015, several high-quality comparative studies using geolocation data have been published (Kramer et al., 2017, Knight et al., 2018).This is very encouraging. Geolocation data still outperform GPS-tracking data in terms of sample sizes and the range of species studied. Thanks to several-year life cycle and daily positioning, geolocators provide suitable data for studying seasonal schedules. In long-shanked birds, these devices can be attached to the legs, letting you define nesting success and incubation patterns.

Solar Geolocation Today

For the moment, solar geolocators remain the cheapest alternative to other tracking devices. They offer an affordable tool for the basic monitoring of long-distance migrations in the light of global change. This won’t always be the case – GPS-loggers will become small, long-lasting and cheap. But until that happens, geolocation data will continue to grow. If treated with care and analysed properly, these data may provide many eye-opening results. With relief we’ve come to the conclusion that our efforts were not wasted, and FLightR will promote the publication of high-quality analyses of tracking data obtained with solar geolocation.